1. Field of the Invention
The present Invention relates, in general, to novel fiber optic sensors and, more particularly, to novel fiber optic sensors which are capable of detecting and/or measuring slowly varying physical quantities for all frequencies from a maximum desired frequency down to zero frequency (DC).
2. Description of the Prior Art
Numerous fiber optic sensors have been developed in the recent past. The more common of these sensors consist of some variation of a two-fiber Mach-Zehnder interferometer. FIG. 1 illustrates a typical interferometer-type sensor for detecting a physical quantity to be measured, such as an acoustic pressure field occuring in a fluid medium. In this type of sensor two optical fibers are provided for establishing separate optical paths of substantially the same length--one defining a detection arm, exposed to acoustic waves, and the other defining a reference arm, isolated from the acoustic waves. Laser light is coupled equally into these arms and propagates therethrough to detectors which provide intensity readings I.sub.1 and I.sub.2. In the absence of acoustic pressure on the detection arm, the two modes propagate through the arms and arrive at the output coupler with a fixed phase difference between them. After being combined in the output coupler and interfering, they produce constant intensities I.sub.1, I.sub.2 on the detectors. However, when subJected to the pressures of an acoustic field, the optical fiber of the detection arm undergoes physical changes (length, diameter and index of refraction changes) which causes a phase delay relative to the mode propagating through the reference arm. When the modes are recombined and interfered, this relative phase shift causes the amplitude of the signals given by the detectors I.sub.1 and I.sub.2 to be changed by a detectable amount. This provides an indication of the magnitude of the acoustic source.
The state of polarization (SOP) of the light emerging from each fiber arm must be correct, and remain so, or the two modes will not completely interfere. Currently available single-mode fibers cannot maintain a specified state of polarization and, as the states of polarization in the fibers change, fringe visibility may fall to zero.
U.S. Pat. No. 4,162,397 issued July 24, 1979 to Joseph A. Bucaro et al. titled "Fiber Optic Acoustic Sensor" discloses a two fiber acoustic sensor wherein acoustic incident on a fiber coil changes its index of refraction at the region of incidence. The index change causes a phase shift in the transmitted light which is detectable to denote the presence of sound waves.
The two-fiber interferometer arrangement is very sensitive to changes in environmental conditions, such as temperature, pressure, air currents, for example, which also introduce phase changes in the propagating light. Because the two fiber arms are physically separate, differential environmental conditions face each and seriously affect the interferometer stability. Also, these environmental perturbations tend to swamp out the desired signal. To overcome these problems, active feedback compensation systems, as illustrated in FIG. 1, have been developed. These compensation systems are based on the principle that environmentally-induced instabilities are low frequency variations. As such, these compensated fiber optic sensors can only detect physical quantities which alternate above a certain predetermined frequency, typically above 100 Hz. Any variations below this frequency are considered to be due to environmental perturbations and hence are compensated out.
The above interferometer can be arranged such that both light paths propagate within the same fiber which may be either multimode or support only a few modes. In this case, the field condition changes the phase of all the propagating modes which interfere to produce a complex interference pattern at the fiber output. Probing this pattern with a suitable aperatured detector gives a signal proportional to the magnitude of the magnetic or acoustic field condition. Unfortunately, this approach is wasteful of light as only a portion of the transmitted light can be utilized If selective excitation at the input is used to excite only two modes of the fiber, then mode conversions due to imperfections can lead to problems. The single fiber interferometer has one advantage in that it does not require beamsplitting devices.
These simpler, single fiber sensors are much less susceptible to environmental perturbations as the light travels in only one fiber instead of two physically-separate fibers. With appropriate design, these sensors can have sensitivities close to those of the two-fiber sensors, leading to higher signal-to-noise ratios. These sensors can detect all the physical quantities that the two-fiber sensor can. However, these sensors still suffer from environmentally-induced instabilities, though less severe than the two-fiber sensor. Provided that the physical quantity is alternating, not necessarily above a certain frequency but different from the environmental instabilities, these sensors can be operated passively free of the environmental instability problems. That is, no active feedback compensation system is required.
Apart from the active, compensated two-fiber sensor which operates above a certain frequency and the passive single-fiber sensor which operates for frequencies different from the environmental instabilities, there is still a great need to detect physical quantities which change at low or zero frequency. For example, accelerations of aircraft rarely change sinusoidally. Constant accelerations are of major importance, as are a number of other constant physical quantities. In short, it is highly desirable that the very high sensitivity of fiber-optic sensors can be exploited at zero frequency, i.e., DC.